# Starting a car with two weak 12V batteries, possible?

Discussion in 'Electronic Basics' started by Indian Summer, Dec 6, 2003.

1. ### Indian SummerGuest

Is it possible to power and automobile starter using 24V on a 12V system,
being careful not to let the 24V bleed into the rest of the car's circuit? I
need a system to start my car in case it runs low on battery. I've been
stranded once and need a way to start the car with just a push of a button
to join two batteries to make the starter crank.

Thanks
Inidi

2. ### John PopelishGuest

While it is certainly possible, it may not be practical. It is much
easier to set up a dual 12 volt battery system that keeps both
batteries charged while driving, but reserves one of the batteries as
a back up for the starter, so that leaving the headlights on or
whatever does not dump both batteries. You just connect both in
parallel with a contactor, when you hit the starter.

3. ### GarethGuest

Yes, it is possible, in fact it is common for breakdown recovery people
to use a 24V battery to start vehicles with 12V electronics. Note that
it is probably easier to connect your two weak batteries in parallel,
this will still give you 12V but the internal resistance of the two
batteries will be lower than either one on its own, so should give more
current to the starter motor. John Popelish's suggestion of keeping a
reserve battery charged is probably better than connecting two weak
batteries in parallel.

The current draw by a car starter motor is very high so make sure that
the wires and connectors you use can handle the current. You should
also keep the high current wires short as the resistance (and therefore
voltage drop) along the wire is proportional to length.

Do you now why your car let you down? If there is a problem with a
specific part, such as the battery or alternator, then I would recommend
that you replace that part rather than mess about with two half worn out
batteries.

I hope that helps,

Gareth

--

4. ### Indian SummerGuest

Is it possible to power and automobile starter using 24V on a 12V
system,
What a great practical idea! So now what kind of diodes ratings should be
used to controll currents flowing only in one way so that it charges but not
drain while driving, except for starting?

Thanks
Indi

5. ### Indian SummerGuest

Is it possible to power and automobile starter using 24V on a 12V
system,
I'd wanted a battery in the trunk since I sometimes forget to turn off the
headlights when exiting or run my portable DC electronics too long. Putting
in the trunk means running a long wire, I have no choice. Let's see what
happens. And thanks, your idea helped.
Indi

6. ### Rheilly PhoullGuest

Don't forget to make that a "Long HEAVY wire"

7. ### DougGuest

I'd just get on of those jumper boxes and through it in the trunk. Yes
you'll have to get out and jump the battery but its the simplest/ cheapest
solution. One of the problems with trying to put a battery in the trunk is
the length of the wire requires a large wire that's hard to handle and
route. There is also the problem of firmly fastening it to the vehicle so
it doesn't become a projectile in the event of an acident or you tip it
over and leaking acid eats the trunk (I've done that, don't ask how I was
17 at the time). The other factor is that repeatly running a car battery
down does them no good.

8. ### Henry KolesnikGuest

It would make more sense to spend your time and money on prevention circuits
and fixing whatever is wrong. I'm 63 yrs old and have had many cars and
problems and many times dreamed of solutions like yours but never found a
practical one, Headlight reminders, timers and other things are much more
practical. Getting two 12 volt batteries in series to start and in parallel
to charge is a big challenge. Depending on how your car is wired you could
zap more than the starter with 24 volts. Adding another alternator to two
batteries in parallel would also be a challenge. Most of the world thrives
on one battery and you can too. Don't waste your time and money. Just my 2
cents worth.
hank wd5jfr

10. ### Ian StirlingGuest

A much simpler idea might be to add a loud warning buzzer that goes off
at 11V or so.

11. ### Stephen J. RushGuest

The RV supply house will probably have switches and relays designed
for this service.

Another possibility is a starter solenoid, the type that mounts near
the battery instead of on the starter motor. This is just a
heavy-duty relay. Connect the main terminals between your spare
battery and the original battery, bypassing the isolator diode.
Energizing the solenoid gives the equivalent of a jump start. You
don't have to worry about disconnecting the main battery, but make
sure that you can't leave the solenoid energized when you drive away.
Not only are those relays designed for intermittant duty, but it would
defeat the purpose of the battery isolator. A push-button or

Remember that the starter motor draws around 100 amps, so use at least
6-guage wire, and don't depend on the car-body ground. Run the
negative lead to the same terminal (usually on the engine block) as
the original battery ground cable.

12. ### James BeckGuest

http://www.painlesswiring.com/catalogframe.htm

Look under battery accessories

13. ### alsGuest

Lots of options.
I have used, in the past,
1) trunk-mounted main battery
use the biggest cable you can get into a channel. Welding cable is great.
2) front or trunk-mounted auxillary battery
Needs a diode isolator big enough to accommodate max charge.
b) starter helper - use an old Ford-type solenoid, switch as necessary.
c) get a (pickup) diesel kit. Available isolated/non-isolated.
3) 24v alternator/24v system
a) cross-connected series/parallel operation, or
b) center grounded, using -12v to power loads

For the trunk-mounts, use a marine plastic housing, and an old seat belt
or other suitable hold-down. Used this in destruction derbies. If you
get hit harder than that, you got other problems.
As already posted, RV kits are available at your local parts store.
For the 24v method, either get a 24v regulator for the existing alternator,
or get a 24v alternator/regulator from an old truck. The (12v) alternator
windings and diodes are (and have been) fine for use at 24v. Got one
running at 48v, but I would not recommend that for your average bear

<als> als_AT_hal_DASH_pc_DOT_org